Weight maintain or loss during pregnancy healthy?

Has anyone has any success with losing weight during pregnancy? I'm 50lbs overweight starting and I'm not trying to loose but I am watching my diet and excersizing now. I gained 8lbs so far (I'm 17 weeks) and keep having visions during my workouts of my baby getting choked by its cord from me bouncing my belly when I bike. My goa is to not gain to much weight and I just want to feel like I'm doing the right thing even though I know being healthier is.. But my mind keeps taking me to a place where I see it being harmful to my baby. I had a miscarriage last year with no known reason so I think that fear of it happening again is still with me.